1. Field of the Invention
The present invention relates to a fuel cell formed by stacking a plurality of power generation cells. Each of the power generation cells includes an electrolyte electrode assembly and separators sandwiching the electrolyte electrode assembly. The electrolyte electrode assembly includes a pair of electrodes, and an electrolyte interposed between the electrodes. The fuel cell has an internal manifold including reactant passages and a coolant passage. The reactant passages and the coolant passage extend through the power generation cells in the stacking direction, and are connected to reactant flow fields and a coolant flow field, respectively.
2. Description of the Related Art
For example, a solid polymer fuel cell employs a membrane electrode assembly (MEA) which includes two electrodes (anode and cathode), and an electrolyte membrane interposed between the electrodes. The electrolyte membrane is a polymer ion exchange membrane. The membrane electrode assembly and separators sandwiching the membrane electrode assembly make up a unit of a power generation cell for generating electricity. A predetermined number of the power generation cells are stacked together to form a stack of the fuel cell.
In the power generation cell, a fuel gas such as a gas chiefly containing hydrogen (hydrogen-containing gas) is supplied to the anode. The catalyst of the anode induces a chemical reaction of the fuel gas to split the hydrogen molecule into hydrogen ions (protons) and electrons. The hydrogen ions move toward the cathode through the electrolyte, and the electrons flow through an external circuit to the cathode, creating a DC electric current. A gas chiefly containing oxygen (oxygen-containing gas) or air is supplied to the cathode. At the cathode, the hydrogen ions from the anode combine with the electrons and oxygen to produce water.
Various sealing structures are used for preventing the leakage of the fuel gas and the oxygen-containing gas in the power generation cell. For example, a sealing structure disclosed in Japanese laid-open patent publication No. 8-148169 uses a conventional O-ring. FIG. 11 shows the sealing structure of Japanese laid-open patent publication No. 8-148169. A membrane electrode assembly 3 includes an anode 2a, a cathode 2b, and an electrolyte membrane 1 interposed between the anode 2a and the cathode 2b. The membrane electrode assembly 3 is sandwiched between the separators 4a, 4b. O-rings 5a, 5b are provided between the separators 4a, 4b around the electrolyte membrane 1.
Typically, in the power generation cell, an oxygen-containing gas flow field (reactant gas flow field) is provided on a surface of the separator facing the cathode for supplying the oxygen-containing gas (reactant gas) to the cathode, and a fuel gas flow field (reactant gas flow field) is provided on a surface of the separator facing the anode for supplying the fuel gas (reactant gas) to the anode. Further, a coolant flow field is provided between the separators for cooling the power generation cells.
The fuel cell has an internal manifold structure in which a fuel gas supply passage and a fuel gas discharge passage (reactant gas passages) connected to the fuel gas flow field, an oxygen-containing gas supply passage and an oxygen-containing gas discharge passage (reactant gas passages) connected to the oxygen-containing gas flow field, and a coolant supply passage and a coolant discharge passage connected to the coolant flow field extend through outer regions of the separators in the stacking direction.
The reactant gas flow field may be connected to the reactant gas flow passages by connection channels formed by seal members. For example, in FIG. 12, a reactant gas flow field 7a is formed on a surface 6a of a separator 6 along a power generation surface, and a reactant gas passage 7b extends through the separator 6 in the stacking direction.
A seal member 8 is provided on the surface 6a of the separator 6. The seal member 8 includes connection sections 8a separately provided at an area connecting the reactant gas flow field 7a and the reactant gas passage 7b. The reactant gas flows through connection grooves 7c between the separate connection sections 8a. A seal member 9 is provided on the other surface 6b of the separator 6 for sealing the coolant flow field (not shown).
The seal members 8 and 9 prevent the leakage of the reactant gas and the coolant. Further, the load balance in the surface of the power generation cell should be uniform, and the load balance should not change depending on the power generation cell in order to achieve the uniform, and the stabilized power generation performance in each of the power generation cells. In particular, the pressure applied to the power generation cell should be kept at the desired level to stabilize the power generation performance. Further, each of the power generation cells should have a uniform space in the flow field so that the cross sectional area of the flow field does not change depending on the power generation cell, and the uniform flow rates of the reactant gas distributed from the reactant gas passage 7b and the coolant distributed from the coolant passage 7d can be achieved.
According to the structure shown in FIG. 12, the connection grooves 7c between the reactant gas flow field 7a and the reactant gas passage 7b are formed by the separate connection sections 8a of the seal member 8. On the other surface opposite to the surface 6a, a section 9a of the seal member 9 extend along the sections 8a continuously. Therefore, when a load in a stacking direction is applied to the connecting sections 8a on the surface 6a and the section 9a which is provided on opposite surface, the connection sections 8a and the section 9a are not deformed uniformly. Thus, the height difference between the connection sections 8a of the seal member 8 and the section 9a of the seal member 9 occurs. Therefore, the cross sectional areas of the reactant gas channels in the seal members 8, 9 are not uniform. Consequently, the seal performance may not be good, and the reactant gas can not be supplied smoothly to the flow field due to the undesirable closure of the gas channels.
When thin metal separators are used, the balance of the line pressure (load) is not uniform, and the separators tend to be deformed in the stacking direction. Thus, the pressure is applied to the sealing surface or the power generation surface excessively or insufficiently. As a result, it is difficult to achieve the desired power generation performance with the simple structure.
FIG. 13 shows a solid polymer fuel cell stack disclosed in Japanese laid-open patent publication No. 2001-266911. For example, an oxygen-containing gas flow field S2 for supplying a reactant gas such as an oxygen-containing gas is formed in a serpentine pattern on a surface of a separator S1. The oxygen-containing gas flow field S2 is connected to an oxygen-containing gas supply passage S3 and an oxygen-containing gas discharge passage S4 which extend through outer regions of the separator S1 in the stacking direction. A packing S5 is attached to the separator S1 for connecting the oxygen-containing gas flow field S2 and the oxygen-containing gas passages S3 and S4, and preventing leakage of the reactant gas to the other fluid passages.
Stainless steel plates (SUS plate) S7 are provided to cover the connection channels S6a, S6b for connecting the oxygen-containing gas passages S3, S4, and the oxygen-containing gas flow field S2. The stainless plates S7 have a rectangular shape, and having ears S7a, S7b at two positions, respectively. The ears S7a, S7b are fitted to the steps S8 formed on the separator S1.
According to the disclosure of Japanese laid-open patent publication No. 2001-266911, the stainless steel plates S7 cover the connection channels S6a, S6b. Therefore, the polymer membrane (not shown) and the packing S5 are not deformed into the oxygen-containing gas flow field S2. The desired sealing performance is maintained, and the significant pressure loss of the reactant gas is prevented.
However, in the structure of Japanese laid-open patent publication No. 2001-266911, the stainless steel plate S7 is attached to each of the connection channels S6a, S6b of the separator S1. The operation of attaching the stainless steel plate S7 to each of the connection channels S6a, S6b is laborious. When several tens to several hundreds of the power generation cells are stacked to form the fuel cell, the attaching operation of the stainless steel plate S7 is very laborious, time consuming, and thus, the production cost is large.
The stainless steel plates S7 are attached to the connection channels S6a, S6b. Therefore, the width of the connection channels S6a, S6b needs to be larger than the width of the stainless steel plates S7. The surface area of the electrode is not used efficiently. It is not suitable to produce a compact and light fuel cell.